1. Field of the Invention
The present invention relates to a cold cathode for discharge lamps, a cold cathode discharge lamp, and a method of manufacturing the cold cathode for discharge lamps that are used in lighting equipment, the backlights for liquid crystal displays, the light sources for general-purpose lighting apparatuses, and the likes.
2. Description of the Related Art
Discharge lamps are essential in industrial fields and everyday life, and account for approximately half of the light sources for lighting equipment. Particularly, the production of cold cathode discharge lamps as the backlight sources for liquid crystal displays and light sources for general-purpose lighting apparatuses has been rapidly growing in recent years.
An example of a cold cathode discharge lamp is a cold cathode fluorescent lamp. In a cold cathode fluorescent lamp, a pair of cold cathodes facing each other are disposed in a glass tube, and an inert gas and a minute amount of mercury (Hg) are contained in the glass tube. When a high voltage is applied to the pair of cold cathodes, discharge starts between both electrodes. The discharge is maintained to excite the mercury, and ultraviolet rays are generated, so that the fluorescent material emits light. Barrier-type cold cathode discharge lamps are also known. A barrier-type cold cathode discharge lamp has an electrode outside the tube forming the discharge space, and the electrode is not in contact with the discharge surface.
Compared with a conventional hot cathode fluorescent lamp, a cold cathode discharge lamp characteristically has a very long service life, while causing less breaking in hot filaments and consuming less emitter materials for electron emission. Because of this, cold cathode discharge lamps are being widely used for the industrial lighting equipment in which replacing the light sources is difficult. Particularly, there is an increasing demand for cold cathode discharge lamps as the backlight sources for liquid crystal displays.
So as to improve the performances of cold cathode discharge lamps, the present inventors have been developed cold cathode discharge lamps using diamond as an electron emitting material of the cathodes (see Japanese Patent Application(Kokai) Nos. 2002-298777 and 2003-132850). Since diamond has a high secondary emission efficiency and a high sputter resistance, a cold cathode discharge lamp with a high emission efficiency and a long service life can be provided.
In such a cold cathode side of a cold cathode discharge lamp, a cold cathode having diamond coating on a metal material of cylindrical shape, cup-like shape, or the like is employed so as to obtain a high discharge current (see U.S. Pat. No. 5,880,559, for example).
However, a cold cathode for a discharge lamp produced with bulk diamond is very costly. Therefore, a diamond film is normally formed on the surface of a metal material, which has various forms, by the CVD (Chemical Vapor Deposition) method, as disclosed in U.S. Pat. No. 5,880,559. By the CVD method, a diamond film can be uniformly formed on a flat metal material.
When a diamond film is formed by the CVD method, it is possible to form a diamond film with uniform thickness on several flat base members. However, uniform film formation is difficult on non-flat base members having cylindrical or cup-like shapes, for example. In U.S. Pat. No. 5,880,559, for example, the inside of a cup-like member and the inside of a tube-like member are coated with diamond. However, by the CVD method, it is very difficult to form a diamond film with uniform in-plane thickness on a non-flat base member.